1. Field of the Invention
The present invention relates to a superluminescent diode and an optical coherence tomography apparatus including the superluminescent diode.
2. Description of the Related Art
As a method for obtaining a tomographic image of an object or a living body, a method called OCT (Optical Coherent Tomography) is known. Since an OCT system (apparatus) is nondestructive and noninvasive, it can be used in, e.g., inspection of industrial products and diagnosis in the field of medicine. Among such OCT systems, a system called SD-OCT (spectral domain OCT) generally uses a spectrometer-based method as disclosed in Japanese Patent Application Laid-Open No. 2009-283736 in which light is emitted from a light source with a broad spectral bandwidth, and the spectrum of beams which interfere with each other is obtained by an OCT optical system. As described in Japanese Patent Application Laid-Open No. 2009-283736, the resolution in a tomographic image increases with an increase in a spectral width to be obtained in an SD-OCT. Accordingly, broadband light sources are required in SD-OCT system applications.
In an OCT optical system, light from a light source needs to be coupled to an optical fiber. The properties required for a light source are thus that the light source has a broad spectral bandwidth and that the light source can be efficiently optically coupled to an optical fiber. Examples of such a light source that is coupled to an optical fiber and that has a broad spectral bandwidth include a semiconductor device called a superluminescent diode (hereinafter also referred to as an SLD).
An SLD is configured to emit light from an edge, like a normal semiconductor laser. The principle of operation of the SLD is that current injection causes a population inversion in an active layer to emit light through stimulated amplification resulting from the population inversion, as in the operation of a semiconductor laser. Note that the SLD is different from a semiconductor laser in that, for the sake of preventing resonance phenomena, the SLD is configured without a pair of reflecting mirrors for resonance. For this reason, in the SLD, light is amplified by stimulated amplification during back and forth propagation or one-way propagation of the light between ends of a waveguide structure which is formed in a semiconductor, and then the light is emitted from an edge.
A plurality of types of structures is available as the structure of an active layer of an SLD. One of the structures is an active layer structure often used in a normal semiconductor laser, as in ELECTRONICS LETTERS, Vol. 32, No. 3, p. 255, 1996. More specifically, the active layer structure includes a plurality of quantum wells, having the same structure, arranged in an active layer. Another one is called an asymmetric multiple quantum well or a modulated multiple quantum well, in which a plurality of quantum well layers different in emission wavelength (ground-state emission wavelength to be exact) is incorporated as an active layer in one waveguide structure. Japanese Patent Application Laid-Open No. 2009-283736 discloses an SLD using two quantum wells different in emission wavelength as an active layer for bandwidth broadening, and the SLD has achieved a wavelength width of 84 nm.
Depending on required properties of an applied system, a specific spectral shape may be required as well as wavelength band. For example, in an SD-OCT system, the spectral shape of a light source as well as bandwidth broadening affects the quality of a final tomographic image.
Since an obtained spectrum is to be converted into a tomographic image by a Fourier transform, a desired spectral shape of a light source may be a unimodal spectral shape (desirably the spectral shape may be analogical to the shape of a Gaussian function). Such a spectral shape inhibits deterioration of an S/N ratio at the time of a Fourier transform or formation of an artifact which does not occur naturally, and thus it allows improvement in the quality of a tomographic image.
As described above, there is an optimum spectral shape for a light source in an OCT system. An SLD serving as a light source is expected to satisfy the requirement.